234 THE KETE MUCOSUM AND THE TRUE SKIN. 



soles of the feet and the eyelids. In this respect its use is mechanical. 

 It serves as a protective covering to the parts it envelops, being thick 

 where pressure and hard usage have to be provided for, and thinner where 

 there is a necessity for motion. It consists of an aggregation of nucle- 

 ated particles adhering together, the deepest being granules, the inter- 

 mediate more perfect cells, which gradually become flattened scales as 

 they are examined nearer the surface. They undergo constant exuvia- 

 tion, and are as constantly replaced from beneath, the superficial ones 

 becoming dry and horny, thus furnishing a resisting tegument, the oper- 

 ation of which is very well displayed by the action of vesicating agents : 

 a watery discharge from the vessels of the cutis soaks through the lower 

 substance of the cuticle, and raises the dry layers above. The chemical 

 composition of these dry scales is the same as that of nail, hair, horn, 

 and is C 48 , H 39 , N 7 , O 18 . 



At one time it was supposed that the rete mucosum, or layer of Mal- 

 pighi, which is the lowest portion of the cuticle, and there- 



liete mucosum * o .... 



and its color- fore resting on the cutis, is a distinct structure. It is, how- 

 ing matter. everj me rely the most recently-formed portion of the cuticle. 

 The netted appearance it presents originates in the eminences of the pap- 

 illary structure below. Many of its constituent particles contain col- 

 oring matter, especially in the dark races. The pigment seems to be 

 produced by the agency of the sunlight and continued high temperature, 

 though it disappears gradually as the cells containing it approach the 

 surface. It yields a very large percentage of carbon. 



Beneath the epidermis is the derma or true skin. It is composed of 

 The derma ^rous tissue, which also serves to connect it with the parts 

 its construe- beneath, blood-vessels, lymphatics, and nerves. In its areolar 

 lon ' tissue both the white and yellow fibrous elements are found, 



the proportion of each varying according to the mechanical function the 

 part has to discharge, the yellow predominating where elasticity is re- 

 quired, and the white where a resistance to pressure. The derma also 

 contains organic muscular fibres, to which its property of corrugation, as 

 in cutis anserina, is due. On different parts it is of different thickness, 

 being thinnest where motion has to be provided for. A deposit of fatty 

 material, lodged beneath, gives it a yielding support. Its outer surface 

 presents a papillary structure, which is the instrument of touch. This is 

 more perfectly developed on the inner surface of the palm of the hand and 

 fingers. The furrowed aspect of the cutis arises from this. A farther 

 consideration of the mechanism and functions of the papillge is deferred to 

 the description of the sense of touch. 



The photographic engraving, Fig. 99, represents a thin section of the 

 epidermis of the foot of the dog. 



The general method of arrangement of the constituent portions of the 



